1. Field of the Invention
The present invention relates to a blood-pressure measuring apparatus having the function of analyzing a shape or form of a pulse wave occurring to a cuff being worn on a living subject. In the present specification, analyzing a waveform means not only analyzing an overall form of a wave but also identifying a specific point on a waveform, such as a peak point or a rising point.
2. Related Art Statement
A blood pressure measurement is carried out by wearing a cuff on a portion of a living subject, such as an upper arm, so as to press the portion, slowly changing a pressure in the cuff, and detecting changes of respective amplitudes of a plurality of heartbeat-synchronous pulses of a pulse wave obtained from the cuff, or detecting Korotkoff sounds, during the changing of the cuff pressure.
The pulse wave obtained from the cuff for the blood-pressure measurement may be utilized such that the form of the cuff pulse wave is analyzed to obtain other sorts of physical information than the blood pressure, such as augmentation index or pulse-wave-propagation-velocity-related information related to a velocity at which the pulse wave propagates in the body of the subject. For example, Japanese Patent Document No. 9-140679 discloses an automatic blood-pressure measuring device having the function of determining a pulse-wave propagation velocity PWV based on a cuff pulse wave.
An augmentation index, generally known as AI, is determined as follows: First, form of cuff pulse wave is analyzed to identify respective peak points of an incident-wave component and a reflected-wave component of the cuff pulse wave. Then, the augmentation index is calculated by dividing a difference between a magnitude of the cuff pulse wave at the time of occurrence of the peak of the incident-wave component and a magnitude of the pulse wave at the time of occurrence of the peak of the reflected-wave component, by a pulse pressure of the pulse wave. Meanwhile, a pulse-wave propagation velocity as a sort of pulse-wave-propagation-velocity-related information is determined as follows: First, form of cuff pulse wave is analyzed to identify a prescribed periodic point on the cuff pulse wave, such as a rising point, a peak point, or a dicrotic notch. Additionally, a heartbeat-synchronous signal detected from a different portion of the subject is analyzed to identify a prescribed periodic point on the heartbeat-synchronous signal. Then, a time difference between the time of detection of the prescribed point of the cuff pulse wave and the time of detection of the prescribed point of the heartbeat-synchronous signal, is determined. Finally, the pulse-wave propagation velocity is calculated by dividing, by the time difference, a propagation distance between the portion where the cuff is worn and the portion where the heartbeat-synchronous signal is detected.
Thus, the cuff worn for carrying out the blood-pressure measurement is utilized to analyze the form of cuff pulse wave and thereby determine the augmentation index or the pulse-wave-propagation-velocity-related information, and accordingly the total number of sensors worn on the subject can be reduced. However, the cuff pulse wave detected from the cuff suffers the problem of low degree of reproducibility or stability. That is, the physical information obtained by analyzing the form of cuff pulse wave cannot enjoy sufficiently high accuracy.
It is therefore an object of the present invention to provide a blood-pressure measuring apparatus which has, in addition to the function of measuring blood pressure, the function of analyzing the form of a cuff pulse wave for obtaining physical information such as augmentation index or pulse-wave propagation-velocity-related information, and which can detect a cuff pulse wave having an accurate form.
The Inventors have carried out extensive studies to achieve the above object, and have obtained the following knowledge: In many conventional devices in which a cuff pulse wave is detected for blood pressure measurement, the cuff pulse wave detected during the blood pressure measurement is also utilized for obtaining physical information such as pulse-wave propagation velocity. For example, the above-indicated Japanese Patent Document teaches that a cuff pulse wave detected when a pressure in a cuff is slowly decreased after having been increased to a pressure value higher than a systolic blood pressure of a living subject is utilized for determining pulse-wave propagation velocity PWV. However, since the cuff strongly presses a body portion of the subject during the blood pressure measurement, congestion and stopping of venous return, each caused by the pressing of the cuff, and vasodilation caused by abrupt increase of blood-flow amount resulting from the stopping of pressing of the cuff, occurs to the body portion, and accordingly the blood-flow amount or the blood-vessel diameter may change during, or immediately after, the blood pressure measurement. In addition, since respective thickness of skin and subcutaneous tissue of the body portion are decreased once during the measurement and are increased again after the measurement, the respective thickness of skin and subcutaneous tissue may change during a certain time after the measurement. The Inventors have found that since the blood-flow amount, the blood-vessel diameter, and the respective thickness of the skin and subcutaneous tissue affect the shape or form of cuff pulse wave, the reproducibility of cuff pulse wave lowers during, or immediately after, the blood pressure measurement. The present inventions have been developed based on this finding.
The above object has been achieved by the present invention. According to a first aspect of the present invention, there is provided a blood-pressure measuring apparatus comprising a cuff which is adapted to be worn on a portion of a living subject to press the portion; a waveform analyzing means for analyzing a form of a first cuff pulse wave which is obtained from the cuff; and a cuff-pulse-wave obtaining means for obtaining, before the cuff presses the portion of the subject for measuring a blood pressure of the subject, the first cuff pulse wave from the cuff so that the waveform analyzing means analyzes the form of the first cuff pulse wave.
According to this aspect, the cuff-pulse-wave obtaining means obtains, before the cuff presses the portion of the subject for measuring the blood pressure of the subject, the cuff pulse wave from the cuff, for analyzing the form of the cuff pulse wave. Therefore, the cuff pulse wave can be obtained in the state in which the blood-flow amount, blood-vessel diameter, skin, and subcutaneous tissue of the portion of the subject are stable, and accordingly the cuff pulse wave enjoys high accuracy and reproducibility.
A time duration by which the cuff pulse wave is obtained before the pressing of the cuff for the blood pressure measurement may be either short or long. However, in order to minimize the time needed to complete the blood pressure measurement, it is preferred that the time duration be not longer than several seconds.
According to a preferred feature of the first aspect of the present invention, the blood-pressure measuring apparatus further comprises a first pulse-wave-detection-pressure keeping means for keeping, before the cuff presses the portion of the subject for measuring the blood pressure of the subject, a pressure in the cuff to a first pre-determined pulse-wave detection pressure, and the cuff-pulse-wave obtaining means obtains, as the first cuff pulse wave, a pressure oscillation occurring to the cuff in a state in which the pressure in the cuff is kept at the first pre-determined pulse-wave detection pressure.
According to this feature, in the state in which the pressure of the cuff is kept at the pre-determined pulse-wave detection pressure, the cuff pulse wave as the pressure oscillation occurring to the cuff is obtained. Therefore, the cuff pulse wave is free from deformation caused by change of the cuff pressure.
Also, preferably, the pulse-wave detection pressure is pre-determined at a value lower than a diastolic blood pressure of the subject, e.g., in a range of from 40 mmHg to 60 mmHg. Since the pulse-wave detection pressure is lower than the diastolic blood pressure of the subject, the cuff pulse wave is free from deformation caused by tension of the cuff. However, since respective diastolic blood pressure values of living subjects have individual differences, the pre-determined pulse-wave detection pressure may not be appropriate for a particular subject depending upon the diastolic blood pressure of the subject, so that the cuff pulse wave obtained by the cuff-pulse-wave obtaining means may be deformed.
According to another feature of the first aspect of the present invention, the blood-pressure measuring apparatus further comprises a blood-pressure measuring device which measures, using the cuff, a diastolic blood pressure of the subject; a pulse-wave-detection-pressure judging means for judging, based on a comparison between the pre-determined pulse-wave detection pressure and the diastolic blood pressure measured using the cuff, whether the pre-determined pulse-wave detection pressure is appropriate; and a second pulse-wave-detection-pressure keeping means for keeping, when the pulse-wave-detection-pressure judging means judges that the pre-determined pulse-wave detection pressure is not appropriate, the pressure in the cuff to a second pulse-wave detection pressure determined based on the diastolic blood pressure measured using the cuff, after a pre-determined time duration has elapsed since the cuff finished pressing the portion of the subject for measuring the diastolic blood pressure, the time duration being so pre-determined as to allow a tissue of the portion of the subject to recover to a condition thereof before being pressed by the cuff, and the cuff-pulse-wave obtaining means obtains, in a state in which the pressure in the cuff is kept at the second pulse-wave detection pressure by the second pulse-wave-detection-pressure keeping means, a second cuff pulse wave from the cuff so that the waveform analyzing means analyzes a form of the second cuff pulse wave.
According to this feature, when the pulse-wave-detection-pressure judging means judges that the pre-determined pulse-wave detection pressure is not appropriate, the pressure of the cuff is kept to an appropriate, second pulse-wave detection pressure determined based on the actually measured diastolic blood pressure measured and, in this state, another cuff pulse wave is obtained from the cuff for analysis of the form of the cuff pulse wave. Thus, the waveform analyzing means can analyze the form of the cuff pulse wave freed of deformation. In addition, the present apparatus can shorten an average overall measurement time as compared with a second aspect of the present invention, described below, according to which a cuff pulse wave used for waveform analysis is obtained, in each case, after blood pressure measurement.
According to a second aspect of the present invention, there is provided a blood-pressure measuring apparatus comprising a cuff which is adapted to be worn on a portion of a living subject to press the portion; a waveform analyzing means for analyzing a form of a cuff pulse wave which is obtained from the cuff; and a cuff-pulse-wave obtaining means for obtaining, after a pre-determined time duration has elapsed since the cuff finished pressing the portion of the subject for measuring a blood pressure of the subject, the cuff pulse wave from the cuff so that the waveform analyzing means analyzes the form of the cuff pulse wave, the time duration being so pre-determined as to allow a tissue of the portion of the subject to recover to a condition thereof before being pressed by the cuff.
According to this aspect, after the cuff finished pressing the portion of the subject for measuring the blood pressure of the subject and further after the tissue of the portion of the subject has recovered to the condition thereof before being pressed by the cuff, the cuff-pulse-wave obtaining means obtains the cuff pulse wave for analyzing the form of the cuff pulse wave. Therefore, the cuff pulse wave can be obtained in the state in which the blood-flow amount, blood-vessel diameter, skin, and subcutaneous tissue of the portion of the subject are stable, and accordingly the cuff pulse wave enjoys high accuracy and reproducibility.
Like the blood-pressure measuring apparatus according to the first aspect of the invention, the blood-pressure measuring apparatus according to the second aspect of the invention may employ a pre-determined pulse-wave detection pressure at which the pressure of the cuff is kept so that the cuff-pulse-wave obtaining means obtains a cuff pulse wave. However, since the blood-pressure measuring apparatus according to the second aspect obtains the cuff pulse wave after the pressing of the cuff for the blood pressure measurement, it is preferred to determine a pulse-wave detection pressure based on a diastolic blood pressure measured in the blood pressure measurement, and obtain a cuff pulse wave in a state in which the cuff pressure is kept at the thus determined pulse-wave detection pressure.
The waveform analyzing means may comprise means for analyzing the form of the cuff pulse wave so as to determine an augmentation index of the subject, or means for analyzing the form of the cuff pulse wave, so as to determine a characteristic point on the form of the cuff pulse wave and thereby obtain pulse-wave-propagation-velocity-related information that is related to a velocity at which the cuff pulse wave propagates in the subject.